This means that Jehu sent a messenger to deliver the letters. AT: "Jehu wrote letters and sent a messenger to deliver them in Samaria" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here, sitting on the king's throne means to be appointed as king. AT: "make him king in his father's place" (See: [[rc://en/ta/man/translate/figs-metonymy]])
"your master's descendants." Here the man that they selected to be king is referred to as Ahab's royal line of descendants. AT: "to defend your master's descendant" or "to defend him" (See: [[rc://en/ta/man/translate/figs-synecdoche]])